There is currently a trend towards maximizing the fuel economy benefits provided by passenger car engine oils (PCEOs). In general, lower viscosity engine oils will provide higher efficiency than higher viscosity engine oils. Attempts have been made to use conventional low viscosity polyalphaolefin base stocks (PAOs) to achieve low viscosity engine oil formulations. Such conventional PAOs, such as conventional PAO 4 cSt, KV100, can be produced by the use of Friedel-Craft catalysts, such as aluminum trichloride or boron trifluoride, and a protic promoter. The volatility requirements for engine oils, however, limit the amount of low viscosity conventional PAO that can be used and the extent to which the viscosity of the engine oil formulation can be reduced. PCEOs that are too volatile can cause high oil consumption problems. High oil consumption is undesirable for consumers as it creates the need for more frequent oil additions and changes.
Others have attempted to formulate PCEOs with other PAOs, such as metallocene-catalyzed PAOs (mPAOs). However, others have not achieved ultra low viscosity engine oil formulations (e.g., kinematic viscosities at 100° C. of 6 cSt or less). For example, US 2009/0181872 discloses lubricating oil compositions for internal combustion engines. The examples include compositions containing low viscosity metallocene catalyzed PAO (mPAO). These compositions, however, have kinematic viscosities at 100° C. of 8.109 cSt or higher. In addition, while these compositions have Noack volatilities (250° C., 1 hr) of as low as 7.6 mass %, the compositions contain mPAO only in amounts of up to 40 wt % of the composition. Additionally, the compositions have CCS viscosities at −35° C. of 3600 mPa·s, or higher, and include a viscosity index improver additive component in the amount of 4.0 mass %.
US 2011/0039743 discloses lubricating oils using a 3.9 cSt “INVENTION” fluid. For example, it discloses 0W-30 and 0W-40 passenger car motor oils, and 5W-40 heavy duty diesel engine oils, using the 3.9 cSt “INVENTION” fluid. These compositions, however, have kinematic viscosities at 100° C. of 10.8 cSt or higher. In addition, while these compositions have Noack volatilities of as low as 7.4 wt % loss, the compositions contain the 3.9 cSt “INVENTION” fluid only in amounts of up to 48.5 wt % of the composition. Additionally, the compositions have CCS viscosities at −35° C. of 5010 cP or higher, and include a viscosity modifier additive solution in the amount of 4.0 wt % or higher. US 2011/0039743 also discloses example compressor/turbine oil, industrial and transportation gear oil, and automatic transmission fluid oil formulations.
WO2011125879, WO2011125880 and WO2011125881 disclose lubricant to compositions for an internal combustion engine comprising: (A) a polyalphaolefin that has a kinetic viscosity at 100° C. of at most 5.5 mm2/s, a CCS viscosity at −35° C. of at most 3,000 mPa·s, and a NOACK of at most 12 mass %; and (B) a mineral oil with a viscosity index of at least 120. WO2011125879 and WO2011125881 disclose that Component (A) constitutes at least 25% of the entire composition by mass. WO2011125880 discloses that Component (A) constitutes at least 10% of the entire composition by mass. WO2011125881 also discloses that the lubricant composition comprises a polyisobutylene with a mass-average molecular weight of at least 500,000. The Tables of WO2011125879, WO2011125880 and WO2011125881 do not indicate the overall kinematic viscosities at 100° C. (KV100) of the compositions, but the compositions contain the 3.458 mm2/s mPAO only in amounts of up to 30% of the composition. Additionally, each of the compositions contain combined amounts of viscosity index improver solution and polyisobutylene solution of 7.0 mass %. While these compositions have Noack volatilities of as low as 9.1 mass %, the compositions have CCS viscosities at −35° C. of 4300 mPa·s or higher.
US 2010/0062954 discloses transmission fluid compositions that contain metallocene catalyzed PAO. These transmission fluid compositions, however, each include viscosity index improver additives, and would possess properties that differ significantly from the passenger car engine oils of the current inventions, such as deposit performance, total base number (TBN), ball rust test performance and sulfated ash levels.
There is thus a need for new, ultra-low viscosity passenger car engine oils with overall kinematic viscosities at 100° C. of from 4 to 6 cSt that achieve greater levels of fuel economy improvement, while meeting volatility performance requirements. Such ultra-low viscosity PCEOs could be categorized as SAE “0W” viscosity monograded oils.
In order to achieve ultra-low viscosity PCEO formulations, high quality, low viscosity PAOs are needed. This demand for high quality PAOs has been increasing for several years, driving research in alternatives to the Friedel-Craft process. Metallocene catalyst systems are one such alternative. In the past, most of the metallocene-based focus has been on high-viscosity-index-PAOs (HVI-PAOs) and higher viscosity oils for industrial and commercial applications. Examples include U.S. Pat. No. 6,706,828, which discloses a process for producing PAOs from meso-forms of certain metallocene catalysts with methylalumoxane (MAO). Others have made various PAOs, such as polydecene, using various metallocene catalysts not typically known to produce polymers or oligomers with any specific tacticity. Examples include U.S. Pat. No. 5,688,887, U.S. Pat. No. 6,043,401, WO 03/020856, U.S. Pat. No. 5,087,788, U.S. Pat. No. 6,414,090, U.S. Pat. No. 6,414,091, U.S. Pat. No. 4,704,491, U.S. Pat. No. 6,133,209, and U.S. Pat. No. 6,713,438. ExxonMobil Chemical Company has been active in the field and has several pending patent applications on processes using various bridged and unbridged metallocene catalysts. Examples include published applications WO 2007/011832, WO 2008/010865, WO 2009/017953, and WO 2009/123800.
Recent research, however, has looked at producing low viscosity PAOs for automotive applications. The current trend in the automotive industry toward extending oil drain intervals and improving fuel economy is driving increasingly stringent performance requirements for lubricants. New PAOs with improved properties such as high viscosity index, low pour point, high shear stability, improved wear performance, increased thermal and oxidative stability, and/or wider viscosity ranges are needed to meet these new performance requirements. New methods to produce such PAOs are also needed. US 2007/0043248 discloses a process using a metallocene catalyst for the production of low viscosity (4 to 10 cSt) PAO basestocks. This technology is attractive because the metallocene-based low viscosity PAO has excellent lubricant properties.
While low viscosity metallocene-catalyzed PAOs possess excellent properties, one disadvantage of the low viscosity metallocene-catalyzed process is that a significant amount of dimer is formed. This dimer is not useful as a lubricant basestock because it has very poor low temperature and volatility properties. Recent industry research has looked at recycling the dimer portion formed in the metallocene-catalyzed process into a subsequent oligomerization process.
U.S. Pat. No. 6,548,724 discloses a multistep process for the production of a PAO in which the first step involves polymerization of a feedstock in the presence of a bulky ligand transition metal catalyst and a subsequent step involves the oligomerization of some portion of the product of the first step in the presence of an acid catalyst. The dimer product formed by the first step of U.S. Pat. No. 6,548,724 exhibits at least 50%, and preferably more than 80%, of terminal vinylidene content. The product of the subsequent step in U.S. Pat. No. 6,548,724 is a mixture of dimers, trimers, and higher oligomers, and yield of the trimer product is at least 65%.
U.S. Pat. No. 5,284,988 discloses a multistep process for the production of a PAO in which a vinylidene dimer is first isomerized to form a tri-substituted dimer. The tri-substituted dimer is then reacted with a vinyl olefin in the presence of an acid catalyst to form a co-dimer of said tri-substituted dimer and said vinyl olefin. U.S. Pat. No. 5,284,988 shows that using the tri-substituted dimer, instead of the vinylidene dimer, as a feedstock in the subsequent oligomerization step results in a higher selectivity of said co-dimer and less formation of product having carbon numbers greater than or less than the sum of the carbon members of the vinylidene and alpha-olefin. As a result, the lubricant may be tailored to a specific viscosity at high yields, which is highly desirable due to lubricant industry trends and demands. The U.S. Pat. No. 5,284,988 process, however, requires the additional step of isomerization to get the tri-substituted dimer. Additionally, the reaction rates disclosed in U.S. Pat. No. 5,284,988 are very slow, requiring 2-20 days to prepare the initial vinylidene dimer.
An additional example of a process involving the recycle of a dimer product is provided in US 2008/0146469, which discloses an intermediate comprised primarily of vinylidene.